Recently, as a semiconductor device is highly densified and miniaturized, a plasma processing equipment is used to perform processing such as depositing, etching and ashing in a manufacturing process of the semiconductor device. In particular, according to a microwave plasma processing equipment which can generate plasma with a microwave, plasma can be stably generated even under a (high-vacuum) condition in which a pressure is relatively as low as about 0.1 to 10 Pa. Therefore, a microwave plasma processing equipment using a microwave having a frequency of 2.45 GHz, for example attracts a lot of attention.
FIG. 18 is a sectional view showing an example of such conventional plasma processing equipment. Referring to FIG. 18, a plasma processing equipment comprises a chamber 1 in which a substrate 11 is housed to be processed, a high-frequency power supply 5 for generating a microwave, and an antenna 3 for emitting the microwave into the chamber 1.
The antenna 3 comprises a slot plate 3c, a slow-wave plate 3b, and an antenna cover 3a. A plurality of slots (openings) are formed in the slot plate 3c to emit the microwave into the chamber 1. The microwave generated by the high-frequency power supply 5 is sent to the antenna 3 through a waveguide 6. A top plate 4 which constitutes a part of a wall of the chamber 1 is provided at the top of the chamber, and a sealing member 14 such as an O ring is provided between the top plate 4 and the wall of the chamber 1. The antenna 3 is provided above the top plate 4.
A table 7 is provided to hold the substrate 11. Furthermore, a vacuum pump 9 is connected to the chamber 1 to exhaust the chamber 1. The chamber 1 is exhausted by the vacuum pump 9 and gas to generate plasma such as argon gas is introduced to the chamber 1 under a predetermined range of pressure.
According to the above plasma processing equipment, a microwave generated by the high-frequency power supply 5 reaches the antenna 3 through the waveguide 6. The microwave which reached the antenna 3 is propagated in the slow-wave plate 3b and it is radiated to the top plate 4 through the slot plate 3c. In the top plate 4, the microwave vibrates in the surface direction and is propagated from the center to the periphery to generate an electromagnetic field in the chamber 1. Argon gas is ionized by the electromagnetic field generated in the chamber 1 and a plasma generation region 22 is formed between the substrate 11 and the top plate 4, whereby predetermined plasma processing is performed on the substrate 11.
It is necessary to irradiate the substrate 11 uniformly with the plasma in the plasma processing equipment. However, since plasma intensity in the center differs from that at the periphery, it is described in Japanese Unexamined Patent Publication No. 2002-299240 that lowering in plasma density at the periphery of the substrate 11 is compensated by forming a top plate 4 into a concave configuration and bringing the substrate 11 and the periphery of the top plate 4 closer, to maintain plasma even in a low-pressure processing, so that stable plasma processing can be implemented.
Furthermore, Japanese Unexamined Patent Publication No. 2003-59919 discloses that a ring-shaped sleeve is formed in a dielectric window so that a plasma excitation region may not directly come into contact with a metal surface of a processing container wall, whereby an uniform plasma density can be provided on a substrate surface.
According to a plasma processing equipment, in order to ensure strength of the chamber in which a pressure is reduced to resist force from the atmosphere, the top plate 4 has to have some thickness in the surface direction. The top plate 4 comprises a dielectric body and a resonant region is formed in the dielectric body by a microwave and a strong electric field is generated to form a standing-wave. An electromagnetic field is generated in the chamber 1 by this standing-wave, so that a plasma density becomes high. The dielectric body has to have a thickness suitable for forming the standing-wave.
FIGS. 19A to 19E are views showing electric field intensity distributions depending on thicknesses of top plates. FIG. 19A shows the electric field intensity distribution when a thickness of the top plate 4 is 22.8 mm in the surface direction and a part designated by slanted lines is a part in which the electric field intensity is high. FIG. 19B shows the electric field intensity distribution when a thickness of the top plate 4 is 27.8 mm, in which the electric field intensity distribution is spread from the center to the periphery as compared with FIG. 19A. FIG. 19C shows the electric field intensity distribution when a thickness of the top plate 4 is 31.6 mm, in which the electric field intensity distribution is spread to the periphery except for the center, which is the most suitable thickness. FIG. 19D shows the electric field intensity distribution when a thickness of the top plate 4 is 32.8 mm, in which the electric field intensity is high only in the center. FIG. 19E shows the electric field intensity distribution when a thickness of the top plate 4 is 37.8 mm, in which the electric field intensity is high in the center.
According to the plasma processing equipment shown in FIG. 18, when a pressure in the chamber or a plasma condition such as a power of the microwave is varied, since an electron density in the vicinity of a plasma surface is varied, a penetration depth of the plasma into a material is varied. When a pressure is lowered, since a diffusion coefficient becomes high, the electron density in the vicinity of the plasma surface is lowered and the penetration depth is increased. Thus, when the plasma condition is changed, since an appropriate thickness of the dielectric body is varied, a resonant region for forming the standing-wave is shifted. Therefore, when plasma is to be generated in an optimal state constantly, there is a problem in which it is necessary to prepare a dielectric body having various kinds of thicknesses according to plasma conditions. In addition, since absorption efficiency of the microwave to plasma is low in a low pressure, it is difficult to generate plasma stably in a low pressure (20 mTorr).